In high-reliability computer disk storage systems, there is a desire to have redundancy in all the physical parts which make up a subsystem to reduce the potential for loss of data and down time upon failure of a part. The need for redundancy is especially applicable to the power supply for the storage system and to the AC main which provides the voltage to the power supply. For example, in conventional system configurations, redundancy has meant the use of at least a second power supply, although the system is capable of operating with only one power supply. As such, in the event of a loss of one of the power supplies, the other will continue to be operational. Moreover, redundancy is further assured by powering these supplies from separate AC mains. Accordingly, in the event of a circuit breaker failure or other loss of an AC main to one of the power supplies, the other AC main will continue to supply voltages to the system through the other power supply. It should be noted that where two AC mains exist, it is not uncommon that they originate from different sources and, therefore, carry differing phases. As such, both mains cannot generally be connected to a same power supply.
As these redundant systems increase in size and capacity, so does the demand for the DC supply current. Accordingly, certain systems require at least two power supplies, rather than a single supply, for supplying DC power to the system. In such systems, redundancy is accomplished by providing at least three power supplies, with the third being the redundant supply. However, in the event a system's redundancy relies on two supplies to be functional, the user is faced with a dilemma. Namely, although it is not uncommon for two separate AC mains to be available for redundancy as discussed, it is uncommon to have a third AC main available for use with a third power supply. As such, requiring a third AC main may cause customer dissatisfaction as well as require an additional unplanned expense. Furthermore, where only two AC mains are available, a redundant system having three power supplies causes customer confusion because the customer is forced to "guess" about which AC main won't fail and which power supply won't fail, in order to connect the same together for implementing the best redundancy technique. And, because of the potential phase problem between the mains (depending on customer configuration and AC main sources), only one AC main (rather than both) can be connected to the third power supply.
Accordingly, an object of the present invention is to reduce the impact upon an electrical system in the event of a loss of a redundant AC main, not only for systems having equal inputs and outputs, but also for where the number of AC main outputs is mismatched against the number of power supply inputs.